I have been looking at any and all info about the TM-1 and KM-1 lately and have found no mention of what the flip-up tab on the TM-1 is for. I still ordered it (procrastinated a day too long before the KM-1 went out of stock) hoping the instructions would explain the tab. Well, I got my TM-1 today and the only mention of the tab was to say it may get in the way of setting the mortise size. So, what the heck is the purpose of that thing?

The "Tab" on the end of the TM-1 is crucial for the tool to be used as a flip stop. It keeps the tool parallel to the fence or reference. You do not use it when measuring a mortise or female gap, but you do need it when it is time for cutting.